1. Field of the Invention
This invention relates to the field of measurement methods for determining the blood flow in a line carrying blood.
2. Description of the Related Art
In patients with renal failure, hemodialysis treatment is one possibility for replacing insufficient kidney. In hemodialysis, blood is withdrawn from the patient through an arterial blood line, purified in a blood treatment element and returned to the patient through a venous blood line. The blood treatment element may be designed as a hemodialyzer, in which blood passes through a first of two chambers separated from by a semipermeable membrane, while dialysis fluid flows through the second chamber. Fluid can be withdrawn from the blood by controlling the pressure ratios in the dialyzer.
It is also possible for the blood treatment element to be designed as a hemofilter. In this case, fluid is only removed from the blood through the membrane, but no fluid passes continuously through the second chamber. Most of the fluid volume withdrawn is returned to the patient by administering replacement fluids.
Such treatment methods require an adequate blood flow of approximately 200 to 450 ml/min to achieve adequate purification of the blood within the treatment, which lasts several hours and is performed approximately three times a week. For this reason, dialysis-dependent patients generally have an arterial-venous fistula or a shunt installed between an artery and a vein. Adequate blood flow develops in this vessel, while at the same time the vessel assumes enlarged dimensions in comparison with the other blood vessels, which is advantageous for puncture with a needle.
The blood flow in such a vessel may vary over time. In particular, a gradual blockage of the vessel may occur due to stenosis. If the blood flow drops below the required blood flow rate in the extracorporeal circulation, the blood purification capacity of the treatment is impaired. In most cases, the vascular stenosis is already so advanced in such a case that it can be corrected only by a procedure such as surgery. Therefore, it would be desirable to learn of such an imminent complication at an earlier point in time, so that other techniques would also be available for eliminating this complication.
A number of techniques have been proposed for measuring the blood flow in a vessel. Methods such as ultrasonic Doppler systems, used independently or with an extracorporeal blood circulation (e.g., Weitzel et al., Am. J. Kidney Dis. 38, 935 (2001)), require additional equipment and handling is complicated. In addition, the measurements must be performed by specially trained personnel.
With another known method, an indicator is infused into a vessel at a constant infusion rate, and samples are taken from the vessel downstream (Kaye et al., Clinical Nephrology 8, 533 (1977)). The fistula flow is deduced from the analysis of the dilution values. This method requires an additional infusion device and a sampling device as well as controlled use of an indicator.
Other systems use the extracorporeal blood circulation of the hemodialysis machine to measure the fistula flow. U.S. Pat. No. 5,866,015 describes a method in which the blood flow rate in the extracorporeal circulation is varied, and the changes in blood temperature in the extracorporeal circulation are measured and analyzed. German Patent 199 17 197 C1 also describes the use of various blood flow rates to then analyze the pattern of the measured pressures in the extracorporeal circulation. One disadvantage of these methods is that controlling different blood flow rates is a complicated process and has a negative effect on the blood treatment. In addition, in the method proposed in German Patent 199 17 197 C1, the fistula flow must also be interrupted for a portion of the measurement.
According to the teaching of European Patent 781 161 B1, in addition to exchanging the accesses on the fistula, it is necessary to vary a physical property of the blood at the outlet end of the extracorporeal blood circulation to generate a blood feature that can be differentiated. The extent of this change is then analyzed in the sense of a dilution curve. This method also requires additional intervention procedures such as injection of an indicator solution or another targeted change in the blood. The analysis involves detection of the entire chronological course of the changes, which must be integrated over the bolus-type changes.
Other methods such as those described in International Patents WO 02/053212 A1 or WO 98/17193 A1 also have in common the fact that it is necessary to detect a targeted change in the extracorporeal blood as well as the variations therein over time.
In addition to describing the determination of cardiopulmonary recirculation, U.S. Pat. No. 5,830,365 describes one possibility for determining the fistula flow with the help of recirculation measurements, but bolus-like changes in blood properties are also induced in a targeted manner and analyzed here for this recirculation measurement.
U.S. Pat. No. 4,894,164 describes a method and a device for measuring and influencing the heat balance of a patient during an extracorporeal blood treatment. A determination of the fistula flow is not provided.